ITV viewers were left in shock and massively freaked out on Monday night as a familiar face appeared in new thriller The Sister looking nothing like the character he's most famous for.

After a lot of hype and countless adverts over the past month, the four part drama finally got going on Monday night, with millions of viewers tuning into see if it lived up to expectations.

The Sister focuses on central character Nathan, played by Years & Years star Russell Tovey, is surprised by an unwelcome face from the past.

Bertie Carvel as Bob in The Sister
EUSTON FILMS FOR ITV THE SISTER EPISODE 1 Pictured: BERTIE CARVEL as Bob Morrow. This photograph must not be syndicated to any other company, publication or website, or permanently archived, without the express written permission of ITV Picture Desk. Full Terms and conditions are available on www.itv.com/presscentre/itvpictures/terms For further information please contact: [email protected] 07909906963

Nathan has never been able to forget the worst night of his life: a party that led to the sudden, shocking death of a young woman.

Only he and Bob, an eccentric old acquaintance, know what really happened… and they’ve resolved to keep it that way. But years later, Bob appears at Nathan’s door with terrifying news that threatens to tear Nathan’s world apart.

And it is the man who plays Bob who got the lion's share of attention on social media as Monday's first episode aired. Five years ago plenty of viewers saw Bertie Carvel in action as sharply dressed and sharp tongued cheating husband Simon in hit BBC drama Doctor Foster, with him returning two years later for its second series.

So they couldn't believe their eyes at seeing him with a straggly beard, unkempt look and an erratic and unnerving demeanour as Bob.

One took to Twitter to exclaim: "I cannot BELIEVE that’s Bertie Carvel on The Sister!!!" and another reacted: "I’ve just found out he’s doctor foster’s husband."

Another viewer, not convinced by his performance as Bob, likened Bertie to an array of people, tweeting: "I cannot take Bertie Carvel’s character Bob seriously. He’s like a mix of Fagin from Oliver Twist, Filch from Harry Potter, Pete Wicks from TOWIE & Captain Jack Sparrow."

The Sister continues on ITV on Tuesday night.