The Pilgrims produced an excellent away performance at the Impact Arena and led courtesy of Nicky Walker's second minute effort.
Walker's goal looked like proving to be the winner until Nathan Hotte headed home a 90th minute corner to haul the Reds level.
Craig Elliott had opted to name an unchanged United XI from the one which drew 2-2 with leaders Bradford Park Avenue at the weekend.
Jay Rollins, making his second successive start, motored away from Hotte to cut the ball back from the right for WALKER to steer home at the near post inside 78 seconds.
Alfreton gradually responded and Ben Davies was well-placed to divert Richard Peniket’s header over the crossbar.
United had a golden chance to double their lead but home keeper Sam Ramsbottom made a fine save with his legs to deny Rollins midway through the first half.
George Willis got all the way behind Tom Platt’s drive from distance, while United’s Max Wright crashed an effort off target from outside the box.
Reece Styche, meanwhile, sailed a free-kick too high from 20 yards after being teed up by a square ball from ex-Pilgrim James Clifton.
Ramsbottom again excelled to keep out Rollins’ angled and deflected drive towards the end of the first period.
Chances were at a premium after the break, with Styche skewing an effort wide for the hosts while Walker saw an effort controversially chalked off for offside as Wright felt the full force of Ramsbottom’s challenge.
Tom Allan planted a free header over the bar following Curtis Bateson’s cross from the right but Alfreton finally levelled in the last minute when HOTTE guided home Jordan Sinnott’s corner delivery.
The major talking point unfolded deep into added time when United debutant Tai-Reece Chisholm whipped in a dangerous ball from the left and Gavin Allott tumbled under Luke Shiels’ challenge, only for referee Aaron Bannister to wave away Boston’s penalty appeals.
Alfreton (4-2-3-1): Sam Ramsbottom; James Clifton, Luke Shiels, Nathan Hotte, Adam Curry; David Lynch, Tom Platt; Curtis Bateson (Jordan Sinnott 79), Tom Allan, Reece Styche; Richard Peniket. Subs (not used): Craig Hobson, Josh Clackstone, Danny Clarke, Tom Nicholson (gk).
Boston (4-4-2): George Willis; Ben Davies, Ryan Qualter, Lewis Gibbens, Ashley Jackson; Max Wright (Jonathan Wafula 82), Brad Abbott, Andi Thanoj, Nicky Walker (Spencer Harris 90); Gavin Allott, Jay Rollins (Tai-Reece Chisholm 72). Subs (not used): Jordan Slew, Sebastian Malkowski (gk).
Referee: Aaron Bannister.