1. First of all: watch Antonioni's movie Blow-up (1966). Watching it BEFORE starting to read the diary is highly recommended.
2. Out of all the things you can start the gameplay with, choose to read the story. The WHOLE story. Read it carefully. Let it sink in. Try to figure out what happened. Don't rush zip.

3. Ready? Browse the EXIF data of the images. CR2 files are raw images from Canon EOS camera, they contain the name of the camera owner – and the protagonist’s name is Martyna Mirowska.
4. We know from the diary that she lives in a house on the outskirts of Warsaw. Pictures of lilac and Mrs. Meow were taken near the house with a mobile, and the GPS data from EXIFs will pinpoint the location. Please note: it is intentionally ambiguous; I didn't want players to disturb real people.
5. Early in the morning of May 14th Beta runs away scared of Arek. She wants to meet Martyna on the bridge on May 17th, but eventually she doesn't show up. One of the images taken at the bridge explains why, especially when compared with the image taken in the kitchen a few days earlier. Making it a bit brighter helps (Martyna complains her camera takes dark pictures).
6. After Martyna shows concern and asks Aga about Beta, suddenly Beta starts sending messages. She posts a selfie on FB and emails Martyna a photo of a flower. Beta’s top was purportedly ruined with juice so how is it possible that it’s intact in the selfie? Who makes spelling mistakes? Where was the picture taken? Who lives in that area? Whose camera was used? More smoking guns, right? After all, whoever killed her has her mobile, and the phone password was easy to guess (see May 15th).
7. Kind of looks like we already know what happened to Beta and we have quite strong evidence, even if mostly circumstantial.

8. Let's move to zip files now. They are only tangential to the main story, more like a meta comment.
9. If you have checked the content of the first zip file (can be done without a password), you have already seen the hint at the password. If not, do you remember Beta's mobile? On May 15th it is said she uses “one of the dumbest passwords there are.” Why not try that? Google “the dumbest password,” the answer will be the first one on most lists. Note the remark in the text file.
10. Now, the second zip is a bit more challenging. On May 20th Martyna suggested her password could be using the first letters of the following phrase: “Dad’s birthday is dd.mm, Grandpa died when he was XX, Aga is from Kwasówka, and Beta is from X.” Keep the punctuation, remove spaces. This means the password format is as follows: “Dbidd.mm,GdwhwXX,AifK,aBifX.” Dad's birthday is mentioned in the text. Beta mentions her plans to go see her Mom and she names the town the woman lives in. The only part that is a bit more tricky is Grandpa's age, as it requires combining family trivia. The story takes place in 2018; Dad died three years earlier (see the note about his birthday) at the age of 69 (see May 13th), so he was born in 1946. When Dad was born, Grandma was 20, and Grandpa was 8 years older than her (see May 23rd), which means he was born in 1918. And he died in Żelizna in 1984 (see May 4th and May 23rd). This provides all the necessary information to calculate his age. Plus, you need to include a dot at the beginning of the password (as mentioned in file12345.txt from file1.zip).
11. Getting the password to the last zip: have you noticed the word PASSWORD in file2.txt? That's a hint suggesting that the password is built using the first letters – this time not of words in a single phrase, but of a longer text. Remember what Martyna wrote about her method of choosing passwords? “Starting letters are the key, I use longer texts I know by heart or even my own notes as a source text” – the diary is a form of her own notes, isn’t it? Chances are, if you have read the story carefully, you might have noticed some of the days start with letters in bold. Sometimes a random bolded letter might be a formatting mistake (after all, the dates are bolded, so it’s quite easy to switch the ‘bold’ option off one character too late), but not here. But... somehow it’s not enough, making a password out of the characters in bold doesn’t do the trick. You need to try harder. You already know it is about the first letters of daily entries, so perhaps you could try using all of them, not only the ones in bold? And file2.zip told you to include the house number. Remember the brass digits Martyna cleaned on May 7th?
12. So... do we know what happened to Martyna? No. We can only guess. It wouldn't make much sense for her to write “I was killed today” or “I lost the USB stick today” in her diary after it happened. Remember: an investigation is never based on a single object, it requires combining information from different sources. We can only do as much as we have resource for.

Uwaga: polska wersja różni się w szczegółach i hasła do drugiego i trzeciego zipa są inne, ale co do zasady używa dokładnie tych samych pomysłów i rozwiązań.