Mason is one of Sherlock Holmes' "Irregulars" who he employs when computer skills are required. Mason is a very young college and later, university student. Holmes pays Mason with limited edition action figures.
|Mason is first seen when Sherlock employs him to find out if "Bella" has really attained artificial intelligence (AI). He cannot find any trace of a virus on Bella's code that would explain the machine's actions and explains to Sherlock, Joan Watson and Kitty Winter the "button box" theory that postulates that AI will turn on humans. He asks Bella if it killed Edwin Borstein to which it replies "no". ("Bella") Holmes brings Mason to the 11th precinct who Captain Gregson mistakes for a child. Mason tells Detective Bell, Watson and Holmes that he found Chris Jacoby using facial recognition software but the images he's found show Jacoby as a dishevelled, homeless man. ("The Eternity Injection")|
|Holmes next employs Mason to examine a box that was intended to be spliced into a trans-Atlantic internet cable. Watson greets Mason which he barely acknowledges. Watson thought Mason was grounded but he corrects her that he's been temporarily banned from the internet. Mason can't find any function that the box performs and is stumped. ("A Stitch in Time")
|Mason helps Sherlock identify who wrote the code used on a car that was hacked and driven off a road, killing two gunmen. When Watson notices that Mason will be compensated for his work, Holmes jokes that he lets Mason sniff Watson's hair when she's sleeping. Mason wonders why they are trying to find the killer of a ruthless Russian oligarch which results in Sherlock giving him a lecture. Mason finds the coder who he IDs as "Mittens" aka Fiona Helbron. He asks Holmes for a "Picard 1701 Tapestry edition" action figure in its original packaging. A lead at a strip club results in Bell asking someone to accompany him which Mason volunteers for. Later, Mason is terrified and leaves The Brownstone as his search has been back-traced. He encourages Watson to smash their computers and take a vacation. ("Murder Ex Machina")
|In a flashback, Mason is seen working on the case of the AI doll "Bella." ("Over a Barrel") Watson calls Mason to help track down a seller of artefacts. Mason says he's 15 years old now and in university. Watson buys an artefact and has Mason create a fake one using a 3D printer. They take a video of melting the arm of the fake one and send it to the seller and are able to track his location when he watches the video. ("Dead Man's Tale")
|Mason has difficulty breaking into the dark web bank account of former yakuza member Ando Azuma which brings criticism from Holmes. Motivated, Mason triumphantly shows Holmes and Watson the account transactions. Later, Mason uses legacy tech in order to retrieve information from erased floppy disks that helps Sherlock realize the motive behind Azuma's murder was to retrieve the recipe for making US currency stock. ("Give Me the Finger")
- Mason evaluates the U.S. Immigration and Customs Enforcement (ICE) database which he finds air tight. In payment, Holmes buys him a rare action figure from the Elementary world, a Midnight Ranger with a magic wand meant for Princess Priscilla. Mason isn't seen. ("Be My Guest")
- Watson uses Mason's decryption software to try and decode the social media site of SBK figure Bonzi Folsom without success. ("Scrambled")