We would start out with Bo and Kenzi in Toronto. There have been funny reports coming in from across the globe about some sort of virus that is causing people to go nuts and kill the people closest to them. After a little digging, the team traces the virus origin.
It’s Beacon Hills.
Lydia Martin was looking for a way to get back at Peter for playing with her mind. She never wanted to hurt anyone else, but when she goes after him, Boyd gets in the way. He’s the first victim, killing both Isaac and Erica before anyone knows what is happening. The virus quickly becomes airborn, infecting anyone with a supernatural basis, of which Beacon Hills has plenty.
When Kenzi and Bo arrive in the town, most of the pack is dead. All that is left is Jackson and Scott, and that’s just because Jackson is somehow immune from his contact with Lydia, and Stiles has Scott locked up in the jail. The town is suffering major losses, including Sheriff Stilinski and Melissa McCall.
Scott got to them before anyone could stop him.
Bo quickly falls under the virus, and Stiles and Kenzi team up with Lydia to try and find a cure before the rest of the world pulls itself apart.
Oh and also Kenzi and Stiles end up running off together after Bo dies. There isn’t much left for them, so they go to start something new together.