That scene from the trailer (the one with Emily Blunt hiding behind a tree that looks as though it's stolen from the best piece of action from 'The Village') is right at the end - but you probably won't have made it that far as life is too short. Never seen so many people leave a cinema half way through a movie, bored and restless.
So, next full moon Lawrence follows his father into a crypt previously hidden in the grounds of the estate (with a load of candles, already burning, dotted around the walls). Inside is his mother's monument - and a passage to an ominous room with more candles and a chair with restraints. His father says something about letting the beast run free, then locks himself in the secret room - and his eyes glow in the dark.
Making his way out, Lawrence transforms into a werewolf, and goes on the rampage.
Next day he is captured by Detective Abberline (who keeps up his Matrix-style "yes Mr Anderson" voice throughout, which is just irritating) and taken to an asylum in London. He is basically tortured for a month, and then as a presentation before an auditorium of experts and city gents he is unveiled during the next full moon. He is soon transformed, breaks loose, kills a few of them and then jumps out a window and away over the rooftops. Next day he wakes up in the sewers and goes to Gwen for protection.
Abberline searches Gwen's house first, but misses out on Lawrence who has returned to the Talbot family mansion.
Lawrence finds Singh, murdered, inside the entrance hall - and then stalks the rooms looking for his father. They finally meet up in the dining room - and Lawrence's memory has played tricks on him all these years - his mother didn't kill herself but was killed by the old man.
His father reveals how that, during a hunting expedition in India, he was dared by his team to go into a cave and find a mysterious ferral boy. The boy bit him, and soon after John discovered that he had a super-healing ability, and the nasty side-effect of turning into a werewolf every full moon. Singh has known his secret, and helped lock him up every month - but now he wants to be free again.
Lawrence shoots him with the shotgun packed with silver bullets - but his father reveals that he took the powder out of the bullets years before. They both turn into werewolves in a ridiculous CGI moment, even worse than Teen Wolf, and fight to the death. A fire enrages the mansion, and Abberline shows up but is bitten in the neck. Finally, Lawrence rips the head clean off his old man.
Gwen is on her way to the mansion. Lawrence gets out and finds her. It looks like there is some humanity in his eyes still - but then he goes for the kill and she shoots him in the chest. As he dies next to her, he transforms back to a man.
As it ends, it's clear that Abberline will now be the next werewolf.