Strax is an occasional companion of the Doctor. He serves as the Butler for Madam Vaxtra.
In the year 4037, Strax was tending to human soldiers at the Battle of Zaruthstra when he was summoned by the Doctor to help in the mission to rescue Amy and her baby at Demons Run, which offered him a chance to once again experience the pleasure of battle. He was seriously wounded in the battle with the Headless monks when one struck him from behind with its sword. As he struggled with what he believed to be his last breaths, Strax noted how he had once longed to die in combat, but now he didn't find it as enjoyable as he had thought it would be. Rory tried to comfort him, saying he was sure he would recover, but Strax reminded Rory that he was a nurse, too. However, he admitted that he had had a good life, and was pleased to live nearly twelve years.
Following his death Strax was brought back to life by Jenny and Madame Vastra, using the same device with which Strax would later effect Clara Oswin Oswald's resurrection. Indeed, the Doctor later explicitly told Oswald that Strax died and had been resurrected. (TV: The Snowmen) However, at the precise instant he regained consciousness at Demons Run — two days after sustaining his fatal injuries — Jenny de-emphasized the severity of his wounds by telling Strax that he had just fainted. This annoyed the Sontaran who had wanted to have the glory of dying in battle. The Victorian couple then offered to take him home with them to 1888 London. At first he declined, but quickly changed his mind.
He went on to assist Madame Vastra and Jenny in Victorian London as part of the Paternoster gang of investigators.
On one occasion the Doctor took Strax for a game of mini-golf on Bouken and left without him. He later returned with Amy Pond while searching for his sonic screwdriver and Strax stood at the door of the TARDIS, waving his golf club at the Doctor. The Doctor claimed that he won all three of their mini-golf games.
In an undetermined year in December, Strax was sent to the Ranskill Gardens meet with Felicity Gregson, a woman who claimed to have evidence of a strange crash behind her house a few nights earlier, whom Madame Vastra was unable to meet with due to her appearance at the Russian embassy. Instead, he ran into Harry, who claimed to have found a woman in his snowman who died. He brought Harry back to Paternoster Row to tell his account to Madame Vastra.
Despite his remarks about Moonites and his belief that they were in alliance with the Rutan Host, Strax showed his tactical expertise on many occasions, such as having built a secret exit inside Vastra's internal chamber, allowing them to escape the fog-like alien creature who was after Harry. He also showed his brute physical strength on several occasions to protect Harry, Jenny, and Vastra. He was eventually put in charge of protecting Harry while Vastra and Jenny fought the fog-like alien invader and discovered that the creature could be killed with snow, which allowed him to save both Harry, Madame Vastra, and Jenny. By the end he'd taken a liking to Harry and offered to recommend him to the Sontaran Greater Military Academy.
Strax was later accompanying the Doctor when he met Clara Oswald. After the Doctor locked Clara in his carriage, Strax tried to persuade him to examine the alien snow that was falling around London. The Doctor, still grieving for the losses of Amy Pond and Rory Williams, refused to get involved and ordered Strax to get a memory worm to erase the last hour of Clara's memory. Strax forgot to put gauntlets on twice and the amused Clara passed them to the Doctor, who retrieved the memory worm himself and put it in a jar. A snowman burst from the snow and attacked the Doctor and Clara followed by several more. They melted when Clara pictured them doing so as the Doctor realized that they were feeding off her thoughts. He then told Strax to take Clara back to the pub where the Doctor had found her, having decided not to wipe her memory because if he did she would forget how to protect herself from the snowmen.
Strax later met Clara when she visited Vastra in order to find out more about the Doctor. After threatening her with obliteration he politely asked for her coat and left Clara with Vastra and Jenny. After Clara persuaded the Doctor to get involved with the word "Pond" Strax went to see if the Time Lord needed any help. The Doctor sent him away after insulting his intelligence but Strax teased him by calling him "Sherlock Holmes".
Strax was later involved in protecting Latimer and his two children when Walter Simeon arrived with a group of snowmen. He displayed his combat expertise by recognizing that the Snowmen were in a defense formation and choosing the most dependable area to make their stand which was Latimer's office. When Clara fell to her death from a cloud, Strax used some alien technology to revive her. After the Doctor and Vastra defeated Simeon and the Great Intelligence, Strax sorrowfully informed them that Clara only had moments left. The Doctor lost another companion that night, but on the day of her funeral he realised that another version of her was living in another part of the universe and rushed off to find her.
In 1893, Mr Thursday came to Vastra so she could look at a photo of his deceased brother, Edmund. Realising Edmund's eyes showed an optogram of the Doctor, the trio headed north, where Jenny infiltrated Sweetville to find the Doctor. Vastra recognized the symptoms of Edmund and others inflicted by the "crimson horror" as that of the repulsive red leech's poison.
Getting lost, Strax asked for the help of the young boy, Thomas Thomas to find Jenny in Sweetville. Strax and Vastra rescued Jenny, the Doctor and a version of Clara from the 21st century from Mrs Gillyflower's pilgrims. Vastra told Strax to leave so he could calm down from the sherbet fancies he ate. Noticing the smokeless chimney at the match factory light up, Strax and Thomas climbed the chimney, which was actually a rocket silo. Seeing his friends threatened by Mrs Gillyflower, Strax shot at her, causing her to fall to her death. Afterwards, Mrs Gillyflower's daughter, Ada crushed the leech Mrs Gillyflower was feeding off of, Mr Sweet.
On a weekend off in Glasgow trying to "destroy" the humans there, Strax received a letter from Vastra inviting him to a conference call in a dream along with Jenny, the Clara from 2013 and Professor River Song in the Library. He asked Archie to knock him out to get there. Vastra showed that the murderer, Clarence DeMarco had said "The Doctor has a secret, you know. He has one he will take to the grave. And it is discovered." along with the space-time co-ordinates to Trenzalore, where his tomb was. The call was interrupted by Jenny's murder at the hands of the Whisper Men. River woke up Vastra and Strax, keeping the link with Clara. To get the attention of the Doctor, the Great Intelligence used the Whisper Men to take Strax, Vastra and Jenny's body to Trenzalore. Jenny was revived and River, unheard to the Intelligence, spoke the password to enter the tomb, the Doctor's name. Inside the tomb, the Intelligence entered the Doctor's timeline, the time winds shattering him into a million pieces, each echo of the Intelligence undoing every one of the Doctor's victories.
Jenny died in the new timeline without the Doctor, while Strax had forgotten his friendship with Vastra and became aggressive, forcing her to kill him. Clara reversed these effects by entering the timeline after the Intelligence, saving the Doctor wherever he went. Strax was revived and apologized for his attack on Vastra who forgave him, knowing it wasn't his fault. The Doctor then decided to save Clara by entering his timeline himself, telling the Paternoster gang that the TARDIS could return them home with the fast return protocol if he didn't return.
Strax arrived at the Thames, along with Vastra and Jenny, to discover a Tyrannosaurus rex had inexplicably arrived in the river. He was surprised, moreover, when the beast threw up the Doctor's TARDIS. There he encountered the Doctor, newly regenerated into his twelfth incarnation, as well as his companion, Clara. Strax led the Doctor back to Vastra and Jenny's house, where he served as a butler, where the others attempted to get the Doctor to sleep off his post-regeneration trauma.
Strax led the rest of the Paternoster Gang to the river again that night, where they discovered that the dinosaur had been reduced to nothing more than smoke and ash through the process of "spontaneous combustion". Vastra, along with the Doctor, took on the case and began investigating several similar deaths that had been plaguing the city. Soon enough, Clara discovered a cryptic ad in a newspaper that led them to Mancini's Family Restaurant, which served as an organ-harvesting factory for a group of Clockwork Droids from the 51st century who had crashed on Earth millions of years before.
Clara and the Doctor investigated the restaurant alone, with Strax, Vastra and Jenny waiting outside for Clara's distress signal. Upon receiving it, the gang cut through the clockwork decoys populating the ground floor of the restaurant and descended into the belly of the droids' crashed ship, where the Doctor and his companion were being accosted by the droids' control node. Their surprise arrival forced the control node to flee, followed closely by the Doctor, in his escape capsule. Meanwhile, Strax and the others fought off the other clockwork droids, only to eventually find themselves overwhelmed and surrounded. They defended themselves by holding their collective breaths, causing the droids to mistake them for fellow droids.
Fearing that he could no longer hold his breath, Strax turned his gun onto himself, willing to sacrifice his own life rather than risk that of his friends. Vastra stopped him, managing to prevent his death but placing the quartet (including herself) in danger again. However, before the group could be killed, the droids were deactivated due to the death of their control node. Victorious, Strax and the others returned home, where Clara was reunited with the Doctor and Strax, Vastra and Jenny returned to life as normal on Paternoster Row.