Tottenham’s decision to stage this tie in Milton Keynes may not have been popular but the makeshift hosts sure delivered drama. Their trip looked to be heading for failure when Isaac Success fired Watford into the lead early in the second half but Spurs turned that round through a late penalty by Dele Alli and a counter-attacking goal by Érik Lamela. There was still time for the former Spurs midfielder Étienne Capoue to increase the suspense by equalising for Watford in the last minute.

But in the shootut Capoue’s effort was saved by Paulo Gazzaniga, who also foiled Domingos Quina’s effort. That left Alli, a Milton Keynes native, to fire in the decisive spot-kick for Spurs. Home, sweet home.

Having done his best to talk up Tottenham’s temporary accommodation 50 miles north of the club’s base, Mauricio Pochettino saw his team walk out into a venue that was about three-quarters full. The stadium was bedecked in Tottenham colours but the giant MK Dons lettering on the seats was clear to see, highlighting the incongruity of this arrangement.

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Still, Tottenham operatives outside the ground tried to generate a novel roadshow kind of atmosphere before kick-off. OK, they tried to flog merchandise to Buckinghamshire residents who might not get many opportunities to visit the club’s official stores. Whatever, the mood at kick-off was pretty good as the spectators who turned up urged the sides to make their journeys worthwhile. Those in white even got a loud “Stand up if you hate Arsenal” chant going just to help them settle in.

And for at least one person in a Spurs shirt this really was a homecoming. Alli was even given the captain’s armband as he returned to the town where he grew up and the ground of the club from which he joined Tottenham three years ago. Locals acknowledged his rise with a special cheer when his name was read out before kick-off. His inclusion was one of eight changes made by Pochettino to Spurs’ starting lineup from Saturday’s Premier League win at Brighton. That was restrained compared to Javi Gracia, who made 10 alterations to the side that has started all six of Watford’s league matches so far this season, with Craig Cathcart the only survivor. The players drafted in included a pair of 18-year-old midfielders, Ben Wilmot and Domingos Quina. Yes, the lineups increased the sense that this was an exercise in improvisation.

Not a very satisfactory one, either. The opening 20 minutes passed almost unnoticed, the main intrigue concerning the roles of Lucas Moura and Érik Lamela, who served as a free-roaming forwards in the absence of Harry Kane, with Alli making his usual darts from deeper. Plodding Spurs struggled to create chances for any of them.

Perhaps inevitably, the first shot of the game was the product of confusion, as a miscommunication in the 23rd minute between Heurelho Gomes and his defence almost allowed a long pass by Alli to drop to Lucas. But Watford recovered quickly enough to block the Brazilian’s shot from 15 yards.

Watford soon threatened at the other end, Marc Navarro twice going close from long range.

Watford might have taken the lead on the half hour if not for a shoddy first touch by Ken Sema and a superb last-ditch tackle by Serge Aurier after Success had opened up the Tottenham defence. Just before half-time a mistake by Ben Davies presented Navarro with another chance to demonstrate his long-range shooting. Paulo Gazzaniga saved again.

Gracia’s pre-match claim that he would treat this like a Cup final was looking dubious but his words at the break must have hit home because his side took the lead within 34 seconds of the restart. Kiko Femenía ran from midfield before feeding Success, who shrugged off a defender and walloped a shot into the net from 12 yards. It was an excellent strike by the Nigerian whose career at Watford has been hampered by injury since he joined two years ago.

Spurs posed little danger until the 55th minute, when Moussa Sissoko clipped a pass to Alli, who volleyed over from 15 yards. Gradually Spurs found more intensity, especially with the arrival of Son Heung-min and Mousa Dembélé. Another substitute, Fernando Llorente, headed over from close range after a cross by Aurier.

The twist came when Alli was upended in the box by Christian Kabasele, introduced three minutes earlier to protect Watford’s lead. Kabasele saw red and Alli netted. Then Lamela finished off a counterattack to seemingly give Spurs victory. But Capoue drew Watford level with a shot from the edge of the box in the last minute that was deflected in.