Spurs looked set to be humiliated when holders Inter raced to a 4-0 lead inside 35 minutes at the San Siro thanks to goals by Javier Zanetti, a Samuel Eto'o penalty - after Heurelho Gomes had been sent off - Dejan Stankovic and Eto'o again.
But Bale almost completed a sensational comeback single-handedly, scoring one of the goals of the season before netting twice in the closing seconds to further enhance his burgeoning reputation.
Despite Bale's exploits, the defeat was a reality check for manager Harry Redknapp, who had vowed to stick to his attacking principles.
After watching his side ship eight goals in three European away games this season, he must surely make them harder to beat on their travels.
Tonight's game was arguably Tottenham's most prestigious since they last rubbed shoulders with the elite of Europe in 1962.
But they looked anything but at home in this company in a calamitous opening 14 minutes that saw them three goals and a man down.
Inter to cut them to shreds in just 70 seconds, easily drawing Alan Hutton out of position and allowing Eto'o to feed Zanetti, who expertly curled the ball beyond Gomes.
Spurs tried to settle but Gomes was sent off six minutes later.
Wesley Sneijder played a delightful ball inside Benoit Assou-Ekotto to Jonathan Biabiany and Gomes came rushing out and brought down the Inter winger.
Referee Damir Skomina inexplicably showed a red card to Assou-Ekotto before his assistants helped him rectify the decision.
Tottenham sacrificed Luka Modric - who had been playing off Peter Crouch in the 'Rafael van der Vaart role' - for substitute keeper Carlo Cudicini.
Eto'o eventually stepped up in the 11th minute to beat the Italian from the spot.
Inter were merciless and, as 10-man Spurs retreated to the edge of their box, a lovely passing move ended with Stankovic slipping the ball past an unsighted Cudicini.
Tottenham had already recovered from such a dramatic collapse in their Champions League rookie season but Inter were anything but Young Boys.
The Italians' perfect start was spoilt when Christian Chivu - wearing Peter Cech-style headgear - was cautioned for an untidy tackle on Aaron Lennon.
Stankovic twice went close to a fourth and Crouch should have pulled a goal back when he headed over Lennon's fine left-foot cross.
But the embarrassment soon resumed, Coutinho given an age to pick out Eto'o's run in behind the visiting defence 10 minutes from the break.
Cudicini got hands on the in-form Cameroon striker's finish but not enough to keep it out. The keeper also spilled shots from Coutinho and Maicon before the break but recovered well.
Inter lost Stankovic to injury in the opening moments of the second half and, when it was briefly 10 against 10, Lennon's latest cross narrowly missed the head of Tom Huddlestone.
Davide Santon eventually restored the hosts to full strength but when the next goal came, it went to Spurs - and it was a beauty.
Bale carried the ball from inside his own half, brilliantly beating two players before finding the corner of the net with a stunning finish.
Normal service quickly resumed and Santon forced a good near-post save from Cudicini.
Sneijder went close to matching Bale's effort before Assou-Ekotto wasted more fine work from the winger with a horrible finish.
The hosts withdrew Chivu for Goran Pandev just past the hour mark, with Spurs following midway through the half when former Inter striker Robbie Keane replaced Crouch.
Inter's final change saw Ivan Cordoba introduced for Biabiany, with Spurs soon responding by withdrawing Huddlestone for the combative Wilson Palacios, who perhaps should have started.
Coutinho went close from outside the box in the closing minutes, Hutton curled over from similar range and Palacios was booked for a poor tackle on Lucio.
What followed was the most incredible of finishes as Bale netted in the 90th and 91st minutes to complete his hat-trick.
The winger produced a carbon-copy strike to his opening goal before scoring again in stoppage-time to set up the briefest of grandstand finishes.