On Beauty and the Beast Season 3 Episode 11, Liam asks Vincent to help him break into the DHS offices so he can destroy a blood sample they have of his. Vincent has no plans to help him out until Liam informs Vincent he sent samples of Vincent's blood to the offices as well and when they do research into the blood, they'll find out about beasts. Vincent reluctantly agrees to go along with Liam's plan, but Cat is apprehensive about what Liam wants out of Vincent.
While Vincent and Liam are breaking into the DHS offices, Cat and Heather do some digging into Liam's ancestry and find out Liam is the beast from 1854 and has been alive for over 200 years. Cat knows immediately that Vincent won't be able to take him down and realizes the only reason Liam wants to get to the blood is because he wants to destroy evidence of him being alive for hundreds of years. Cat and Tess quickly go to the DHS offices to stop Liam from destroying the blood, but they are too late. Liam is able to run away and realizes Vincent will never be on his side. Cat and Heather clue Vincent in on Liam's lineage and he's surprised to learn a beast can live so long.
Meanwhile, JT finally comes clean about the serum poisoning, but reasons he didn't want anyone to know because he wants them to focus on taking down Liam.
Don't feel bad if you missed out. You can watch Beauty and the Beast online via TV Fanatic and catch up on the entire season before the finale.