The Pilgrims led 3-0 at half-time, courtesy of strikes from Reece Thompson, Jamie McGuire and Ashley Hemmings and although Craig Lindfield and Jeff King reduced the arrears, Hemmings added his second to seemingly but the game back to bed.
The Red Rebels had different ideas though and Gerard Garner and Lindfield struck in an amazing finale to restore parity at 4-4.
The match had survived another bout of wintry weather to proceed and was played in a bizarre mixture of glorious sunshine and sporadic blizzards.
The Pilgrims should already have led when Stephen Brogan’s cross from the left was slammed over the bar at the back post by Jay Rollins.
United’s third minute breakthrough was orchestrated by the recalled McGuire, who clipped THOMPSON into the clear and the Guiseley loanee turned Sam Baird inside out before rifling how low to Lloyd Allinson’s right.
The hosts doubled their advantage five minutes later when McGUIRE sailed home a low effort via a deflection off Danny Racchi after Hemmings’ corner had only been partially cleared.
United were awarded a penalty on the half-hour mark when Salford City loanee Jake Beesley was upended by Baird and HEMMINGS did the rest from the spot.
Boston had a chance to make it four at the start of the second half but Rollins motored into the box and drilled wildly off target from 12 yards.
The visitors took full advantage and reduced the arrears seven minutes in when Joel Senior crossed for LINDFIELD to work a yard of space to hook the ball past Jon Stewart.
Beesley, Thompson, Brad Abbott and Ryan Qualter all saw chances come and go before FC United cut the deficit even further through Bolton Wanderers loanee KING just three minutes after stepping off the bench. Fellow sub Jamal Crawford had neatly assisted.
The Pilgrims eased their nerves by quickly re-asserting their two-goal lead when HEMMINGS sailed home a fantastic, angled free-kick from wide on the United right.
Beesley rattled the crossbar as the Pilgrims chased a fifth goal but it was to prove a major turning point as the Red Rebels launched their rousing comeback.
Fleetwood Town loanee GARNER use a combination of head and shoulder to loop an effort into the top corner with two minutes to play and the visitors’ amazing riposte was completed two minutes into added time when LINDFIELD touched home a corner from close-range.
Boston (4-4-2): Jon Stewart; Wes Atkinson, Ryan Qualter, Jordan Keane, Stephen Brogan; Jay Rollins (Jonathan Wafula 75), Brad Abbott, Jamie McGuire, Ashley Hemmings (Gregg Smith 85); Reece Thompson, Jake Beesley. Subs (not used): Kabongo Tshimanga, Benny Igiehon, George Willis (gk).
FC United (3-5-2): Lloyd Allinson; Matt Hughes, Scott Kay, Sam Baird (Jamal Crawford 46); Joel Senior, Connor McCarthy (Jeff King 68), Danny Racchi (Gerard Garner 83), Steve Irwin, Luke Higham; Tom Greaves, Craig Lindfield. Subs (not used): Kieran Glynn, Jordan Fagbola.
Referee: Declan Bourne.
Attendance: 1,052 (175 visitors).