HJK stunned their Norwegian opponents by taking a 1-0 lead in the first leg in the Finnish capital last week.
Match preview
That defeat was just Molde's third since the end of April, and their first in seven games.
Molde were, and remain, heavily fancied to progress given that they entered the first leg on the back of four straight wins where they scored 14 goals.
They responded to that setback on the continent by beating Stabaek 1-0 at the weekend to keep up their impressive domestic form.
A poor start means Champions League football looks unlikely next year though, so they must ensure they do not slip up in the early stages of qualifying.
After cruising to the title 18 points clear of Bodo/Glimt last season, Molde now find themselves nine points adrift, with only the champions eligible for a spot in Europe's top competition in the Eliteserien.
It would be a remarkable achievement should they make it all the way to the groups though, as Molde have only accomplished that once in their history, back in 1999-00.
The club did reach the groups of last season's Europa Conference League though, and that seems a very feasible target.
Dropping into the Europa League would also be an acceptable outcome, and their last 16 appearance in 2020-21 remains their best ever showing in a UEFA competition.
HJK have been waiting even longer to make another Champions League group stage appearance, but have given themselves a great opportunity to keep the dream alive.
Despite being regular Finnish champions, HJK have only safely navigated qualification once before, back in 1998-99.
Another ominous stat is that HJK have been eliminated at this stage in four of the last seasons.
Youngster Topi Keskinen's solitary first-leg goal saw HJK take a home 1-0 lead, like what they managed in the previous round against Larne.
Toni Korkeakunnas's side only scraped through, with Larne's Shaun Want's extra-time own goal being the decisive moment as HJK went through 3-2 on aggregate.
They do not bring much away form into this clash either, as a win at SJK Seinajoki at the weekend was just their second in nine games.
This season, HJK have won just 14 points from a possible 27 in the Veikkausliiga, as their crown looks in slight danger.
HJK won the title by just a point last season to claim this Champions League berth, and with only four games left of the regular season, one point again separates the top three.
Molde Champions League form:
L
Molde form (all competitions):
D
W
W
W
L
W
HJK Helsinki Champions League form:
W
D
W
HJK Helsinki form (all competitions):
L
W
D
W
W
W
Team News
Martin Ellingsen returned from injury to appear off the bench for Molde at the weekend.
Manager Erling Moe made six changes to the side which lost in Helsinki, and could revert back to a similar side despite that defeat.
Ola Brynhildsen is likely to return in attack, while Martin Linnes and Kristoffer Haugen should start as wing-backs.
HJK goalkeeper Dejan Iliev went off injured for the second time in a month early in their win over SJK at the weekend.
Eighteen-year-old stopper Alex Ramula replaced him off the bench, but Jesse Ost, who started five of the six games prior, is most likely to come in.
Atomu Tanaka and Kevin Kouassivi-Benissan were the only other two first-leg starters who were rested at the weekend, and both could return.
Veteran Perparim Hetemaj was sent off in that win, but will still be available for European selection.
Molde possible starting lineup:
Karlstrom; Linnes, Tiedemann, Bjornbak, Hagelskjaer, Haugen; Eriksen, Mannsverk, Breivik; Kitolano, Brynhildsen
HJK Helsinki possible starting lineup:
Ost; Kouassivi-Benissan, Toivio, Tenho, Raitala, Soiri; Lingman, Kanellopoulos; Tanaka, Radulovic, Keskinen
We say: Molde 2-0 HJK Helsinki
Despite a fantastic win on home soil last week, HJK may find it tough to hold on to their lead in Norway.