Cardiff Blues looked to be heading for defeat at BT Sport Cardiff Arms Park as they trailed Connacht 17-11 going into stoppage time.
But a quite incredible late rally saw Blues keep the ball alive time and time again before – after a tense and nerve-shredding eight minutes – it was Tuculet who raced in to a quite deafening reception.
Then the stadium was to fall silent as Rhys Patchell lined up the conversion and showed nerves of steel to slot the extras and secure a massive home win to a thundering ovation.
Earlier Tuculet had grabbed a second half try to cancel out an early opener from Jake Heenan in a pulsating clash.
Lock Aly Muldowney looked to have won the game nine minutes from time when he stretched over.
However, the late drama was only just beginning as Blues staged the most remarkable of comebacks and dug deep to claim another league win.
Man-of-the-moment Tuculet had been in the thick of the action from the opening minute as the wing kicked ahead in the earlier stages but his grubber was well collected by Tiernan O’Halloran who ran the ball back.
The tackles were rattling in as a huge collision between Rodney Ah You and Taufa’ao Filise shook the very foundations of the stadium as both teams looked to assert early dominance in the collisions.
The Irish province thought they had gone ahead when a cross field kick from Craig Ronaldson was plucked out the air by O’Halloran and touched down. But, after consulting with the TMO, the score was chalked off.
However, there was no stopping the try moments later from a driving lineout as Heenan touched down at the back to put Connacht ahead.
A clever chip ahead from Gareth Anscombe almost came off, but the fly half was pulled back for a knock on as he tried to collect the bouncing ball, while the forwards went to work with Kristian Dacey, Lou Reed and Taufa’ao Filise working the fringes.
It was often breathless rugby from both sides, but a lack of accuracy from Blues was haunting them as they coughed up ball in promising positions.
Ronaldson and Patchell exchanged penalties before a stirring end to the first half from Connacht.
A huge let-off for Blues saw Aly Kearney brush off tackles and rattle down the middle. Support runners were soon up with him and only a knock on denied them as they menacingly built up a head of steam.
Then a scrum against the head saw Connacht come again, but spilled ball, as the forwards looked to make the surprise possession count, cost them dearly.
Blues needed to kick on in the second half and showed early dominance as Gavin Evans crashed down the middle and Rhys Williams took ball on.
A neat show and go from Anscombe almost unlocked the well-drilled Irish defence before a high tackle on Cuthbert and the kick to the corner saw Lloyd Williams have a try chalked off short.
Filise was next to set Blues on the front foot as he bulldozed up the field and sent men crashing. The ball was flipped in field to Anscombe and his delicious delayed pass sent Cuthbert over.
However, after several minutes of deliberation, yet another try was disallowed for crossing.
The no score seemed to fire up Cuthbert and moments later he produced a sublime break as he tore past flaying men and only a last ditch tackle denied him a superb solo try.
Again came Blues and again Cuthbert broke past men on the opposite flank. And this time, when the ball was flung wide, there was no stopping opposite wing Tuculet who drew the final man, spotted a gap and dived over.
The home side now had their tails up and Sam Hobbs, Josh Turnbull and Navidi were all hungry for work and a penalty saw Patchell nudge Blues into a point lead.
Connacht, though, were not to be denied and produced their best spell of the second half as they kept the ball alive and were rewarded when Muldowney stretched over.
That looked to be enough as Blues tried with no joy, despite a yellow card to Mick Kearney, to force an opening in the closing stages.
But then came the eight minutes of injury time and the most memorable of late drama. A night few at the BT Sport Cardiff Arms Park will ever forget.
07m Heenan t Ronaldson c 0-7
23m Ronaldson p 0-10
25m Patchell p 3-10
56m Tuculet t 8-10
65m Patchell p 11-10
71m Muldowney t Ronaldson c 11-17
80m Tuculet t Patchell c 18-17
Cardiff Blues: 15 Rhys Patchell, 14 Alex Cuthbert, 13 Cory Allen, 12 Gavin Evans (Isaacs 72), 11 Joaquin Tuculet, 10 Gareth Anscombe, 9 Lloyd Williams; 1 Sam Hobbs (Davies 75), 2 Kristian Dacey (R Williams 17-24, 38), 3 Taufa’ao Filise (Andrews 65), 4 Jarrad Hoeata, 5 Lou Reed (Paulo 67), 6 Josh Turnbull, 7 Ellis Jenkins (Cook 67), 8 Josh Navidi
Replacements: 16 Rhys Williams, 17 Thomas Davies, 18 Scott Andrews, 19 Filo Paulo, 20 Macauley Cook, 21 Tavis Knoyle, 22 Gareth Davies, 23 Tom Isaacs