An old team member, Mark Collins, of Scorpion uses Walter as his one phone call of sorts once he gets caught near a nuclear reactor site. No one besides Walter is happy to see Mark, although it's not quite clear if even Walter is happy.
There was some bad blood between Mark and the team. Specifically, Walter and Mark weren't good for each other. They only focused on work, and not only the essentials of life like food and sleep. The last time Mark and Walter went on a bender, Happy was the one who had to pull Walter out of it, which was not an easy task.
The nuclear reactor is malfunctioning, but no one knows it. Walter makes an executive decision to take on the case, and he claims that Mark needs to be there to help. Walter publicly sides with Mark over Happy after Mark made some not so nice statements about Happy's intelligence and motives. Happy decides to run her program anyway, and ends up making things a lot worse.
The nuclear reactor goes into containment mode, and it's a mad dash to escape. Unfortunately, Gabe does not make it out. He's trapped, and to make matters worse, there's radioactive steam blowing out of a pipe. With Mark's help, the team is able to get Gabe to safety before he is exposed to too much radiation.
It gets down to the wire. With ten minutes left to fix the reactor, Walter realizes that Mark had been setting everything up in his favor. The team has to go to Mark's house in order to save the day. The one person standing in Walter's way is Mark. It comes down to the wire, but the team manages to save the day and celebrate by cooking Italian with Paige and Ralph.