Despite not topping the seed table, he achieved the highest score of the series by amassing 132 points in his seventh heat game, beating Stephen Read's high by one point. "The Moose", as he was popularly known, garnered a total of 828 points and qualified to return to the show in June for the series finals as #3 seed.
In the quarter-finals, Rosser prevailed over Wesley Jardine in a very close match, picking out the nine-letter word ETHNICIST and sneaking through on the unsolved crucial conundrum POLICECAR. He then achieved an upset in the semi-finals, knocking out #2 seed Jeff Clayton 88 – 84 by spotting that LOSTMYCAP became CYTOPLASM. Only in the grand final did Rosser ever suffer a defeat, going down 108 – 58 to the Apterite and 900-club member Stephen Read in the last game to have been played on the long-lived "original" blue Countdown set. His runner-up finish arguably makes him among the strongest non-Apterites to appear on Countdown during the 2010s.
|#||Date||Type||Contestant 1||Score||Contestant 2||Guest||Lex||Max|
|6586||19/05/2017||P||Amber Cotton||36 – 83||Moose Rosser||Chris Hollins||Susie Dent||120|
|6587||22/05/2017||P||Moose Rosser||90* – 80||Michael Hooker||Chris Hollins||Susie Dent||127|
|6588||23/05/2017||P||Moose Rosser||104 – 35||Bob Saunders||Chris Hollins||Susie Dent||123|
|6589||24/05/2017||P||Moose Rosser||106 – 44||Ellie Foster||Dan Walker||Susie Dent||127|
|6590||26/05/2017||P||Moose Rosser||111 – 76||Peter Sheridan||Dan Walker||Susie Dent||147|
|6591||29/05/2017||P||Moose Rosser||119 – 33||Sam Green||Dan Walker||Susie Dent||144|
|6592||30/05/2017||P||Moose Rosser||132 – 30||Gareth Davies||Dan Walker||Susie Dent||136|
|6593||31/05/2017||P||Moose Rosser||93 – 71||Michael Metcalf||Dan Walker||Susie Dent||158|
|6610||26/06/2017||QF||Moose Rosser||81 – 71||Wesley Jardine||Colin Murray||Susie Dent||136|
|6613||29/06/2017||SF||Jeff Clayton||84 – 88||Moose Rosser||Colin Murray||Susie Dent||121|
|6614||30/06/2017||GF||Stephen Read||108 – 58||Moose Rosser||Colin Murray||Susie Dent||121|