No spoilers in this review, but a brief summary of my impressions about the film: The movie was better than I expected, and I was a bit surprised by the low 4.8 rating (at the time of writing) on IMDb.
Granted, it's not a very good film, but it's not that bad either: The dialogue is OK and it avoids many of the narrative clichés you often see in low-budget films (even big-budget films as well), and the acting is (in general) more than decent enough (more about that later).
Thumbs up to the screenwriters for at least attempting to add some depth to the film: At first you may get the impression that this is some sort of action-film, but it's more like a thriller with some underlying "family" drama (with focus on father/"stepmother"-daughter relationships).
I was less enthusiastic about the pacing of the last half of the movie: I think the film could have been shortened a bit, to add more focus towards the end. I also felt that the foreign accent of Tally was a bit distracting, even though English was not the native language of her character (or the actress herself).
Peter Stormare was, as usual, quite funny. It's almost over-the-top, but he somehow manages to sell the performance because it seems to fit his character.
Maisie Williams is a young and very talented actress. Her character starts off as an annoying, bratty teen, but the turn of events help to bring her into a more sympathetic light (a lesser script could easily have made the character much more two-dimensional and clichéd). Kudos to Maisie for bringing her character to life, even though she didn't have that much material to work with.
However, one reviewer here wrote that "she does not have much acting skill. ... Maisie mesmerizes the audience with her eyes. People mistaken it for acting".
The reviewer couldn't be more wrong, in my opinion: Good acting is not only how you act or perform, it's about how you *RE*act. And Maisie's ability to communicate and express emotions via her eyes is one of the very reasons why she's so talented. She listens and reacts. Maisie's performances are also emotionally honest and "real", and it's not often I see these qualities in young actors in their first screen roles.
Overall, I think this film passed the "watch clock" or "falling asleep" tests: In general, if a film manages to entertain me enough to avoid these things from happening, then it usually deserves at least 6/10. Heatstroke started of well, and although it started to slow down towards the end, it still left a positive impression.